Nuclear physics Flashcards
History of nuclear physics
- Democritus - came up with word ‘atomos’ (atoms)
- Joseph Priestley and Antoine Lavoisier; Age of Enlightenment.
- John Dalton; father of modern atomic theory
- JJ Thompson: Plum Pudding diagram
- Ernest Rutherford - Rutherford’s Scattering experiment; firing alpha particles at a very thin gold foil (one atom thick) in a vacuum. Scintillator (machine which lights up when atoms hits it) is placed behind.
About 1/8000 particles deviated away from in line with gold foil. 1/15-20000 not detected.
This is because some bounced back - something very small and dense must be at the center of the atoms. It must be very positive to bounce back from a positive alpha. This was the discovery of the nucleus. Particles which deviated away from the gold foil had a slight deflection as they were slightly repelled by the +ve nucleus.
Conclusions:
Nucleus is tiny, positive and most of mass of atom is concentrated.
Most of atom is empty space.
The closer things are to one another…
…the greater the force of repulsion
When is all kinetic energy transferred to electric potential energy
Graph for when electrons are attractive/ repulsive
Draw the diagram and graph showing electron diffraction
Half life def
The time taken for the number of unstable nuclei to fall to half its original value
What type of nuclear substance is good for inserting into the body and why
Technetium 99:
- suitable half life of 6 hours, so technetium will have lost radioactivity in a small enough amount of time.
- non-toxic
- gamma emitter
- suitable energy of gamma rays: not too energetic ( would cause damage), but energetic enough to measure on a gamma camera
What factors must be considered when choosing a nuclear substance to inject into someone
- non-toxic
- gamma emitter
- suitable energy of gamma rays
- suitable for job you wants (e.g will it go to e.g kidneys)
Draw an energy level diagram for molybdenum to technetium to rubidium
Half life def
The time taken for the number of unstable nuclei to fall to half its original value
What is one atomic mass unit (1u) equal to
1/12 of the mass of a neutral carbon atom
What is the mass defect/ binding energy
the difference between the mass of a nucleon in theory and the actual mass measured
- equal to The energy released when the nucleus forms
- equal to the energy required to separate the nucleons in the nucleus (typically in MeV)
What is the highest binding energy per nucleon (from the binding energy curve) possible and for what element is this from
8.8 MeV - from Iron 56
What is the most stable element and why
Iron 56 - this element requires the largest amount of energy to separate the nucleons in the nucleus (highest binding energy per nucleon)
Draw and label a diagram of the binding energy table
what is the difference between stable and unstable nuclei
- The most stable nuclei have roughly the same number of protons as neutrons
- If there were too many protons, then the repulsive force created by them all having the same positive charge, would cause the nucleus to repel when it becomes very large
- Therefore, if a nucleus has an imbalance of protons or neutrons, it is more likely to decay into small nuclei until it gets to a stable nucleus with roughly the same number of each
what does it mean and what is the issue with a nucleus being too heavy
- If an nucleus is too heavy, this means it has too many protons and neutrons
- The forces in the nucleus will be weaker in keeping the protons and neutrons together
- This can also cause the nucleus to decay
what happens to the mass number and atomic number in alpha decay
The mass number decreases by 4
The atomic number decreases by 2
Describe alpha or beta decay and draw a diagram of alpha decay
what happens to the mass number and atomic number in beta decay
- mass number of the decaying nuclei remains the same
- Electrons have an atomic number of -1
This means that the new nuclei will increase its atomic number by 1 in order to maintain the overall atomic number before and after the decay
what happens to the mass number and atomic number in gamma decay
Gamma decay does not affect the mass number or the atomic number of the radioactive nucleus, but it does reduce the energy of the nucleus
mass defect def
The difference between an atom’s mass and the sum of the masses of its protons and neutrons
what does the mass defect imply about the difference in mass between separated nucleons and a system of bound nucleons
A system of separated nucleons has a greater mass than a system of bound nucleons
binding energy def
The amount of energy required to separate a nucleus into its constituent protons and neutrons
nuclear fusion def
The fusing together of two small nuclei to produce a larger nucleus
describe the forces involved in nuclear fusion
- For two nuclei to fuse, both nuclei must have high kinetic energy
- This is because nuclei must be able to get close enough to fuse
- However, two forces acting within the nuclei make this difficult to achieve:
- Electrostatic Repulsion - Protons inside the nuclei are positively charged, which means that they electrostatically repel one another
- Strong Nuclear Force - The strong nuclear force, which binds nucleons together, acts at very short distances within nuclei. Therefore, nuclei must get very close together for the strong nuclear force to take effect
why must fusion happen in an extremely hot environment
because it takes a great deal of energy to overcome the electrostatic and strong nuclear forces
nuclear fission def
The splitting of a large atomic nucleus into smaller nuclei
describe how nuclear fission occurs and how a chain reaction may occur
- Fission must first be induced by firing neutrons at a nucleus
- When the nucleus is struck by a neutron, it splits into two, or more, daughter nuclei
- During fission, neutrons are ejected from the nucleus, which in turn, can collide with other nuclei which triggers a cascade effect
- This leads to a chain reaction which lasts until all of the material has undergone fission, or the reaction is halted by a moderator
why do nuclei with a low binding energy per nucleon tend to undergo fusion
- they are the most unstable and likely to undergo fission
- Repulsive electrostatic forces between protons begin to dominate, and these forces tend to break apart the nucleus rather than hold it together
why do nuclei with a high binding energy per nucleon tend to undergo fusion
- have weaker electrostatic forces and are the most likely to undergo fusion
- Attractive nuclear forces between nucleons dominate over repulsive electrostatic forces between protons
does fusion or fission release more energy
Fusion releases much more energy per kg than fission
when does induced nuclear fission occur
When a stable nucleus splits into small nuclei from the bombardment of a slow-moving neutron
what are neutrons involved in induced nuclear fission also known as and what are their characteristics
- thermal neutrons
- Thermal neutrons have low energy and speed meaning they can induce fission
- This is important as neutrons with too much energy will rebound away from the nucleus and fission will not take place
how many thermal neutrons are needed for a chain reaction
just one
critical mass def
The minimum mass of fuel (fissionable material) required to maintain a steady chain reaction
what happens when more or less of the critical mass is used
Using less than the critical mass (subcritical mass) would lead the reaction to eventually stop
Using more than the critical mass (supercritical mass) would lead to a runaway reaction and eventually an explosion
what is the purpose of a moderator
To slow down neutrons
how does a moderator work
- The moderator is a material that surrounds the fuel rods and control rods inside the reactor core
- The fast-moving neutrons produced by the fission reactions slow down by colliding with the molecules of the moderator, causing them to lose some momentum. The neutrons collide elastically.
- The neutrons are slowed down so that they are in thermal equilibrium with the moderator, hence the term ‘thermal neutron’
- This ensures neutrons can react efficiently with the uranium fuel
- Graphite and water are commonly used for moderators
purpose of a control rod
to slow down neutrons
how does a control rod work
- The number of neutrons absorbed is controlled by varying the depth of the control rods in the fuel rods
- Lowering the rods further decreases the rate of fission, as more neutrons are absorbed
- Raising the rods increases the rate of fission, as fewer neutrons are absorbed
- This is adjusted automatically so that exactly one fission neutron produced by each fission event goes on to cause another fission
- In the event the nuclear reactor needs to shut down, the control rods can be lowered all the way so no reaction can take place
- Boron and cadmium are commonly used for control rods
purpose of the coolant
To remove the heat released by the fission reactions
how does a coolant work
- The coolant carries the heat to an external boiler to produce steam
- This steam then goes on to power electricity-generating turbines
how are nuclear reactors stopped from giving workers exposure to radiation
- The fuel rods are handled remotely ie. by machines
- The nuclear reactor is surrounded by a very thick lead or concrete shielding, which ensures radiation does not escape
- In an emergency, the control rods are fully lowered into the reactor core to stop fission reactions by absorbing all the free neutrons in the core, this is known as an emergency shut-down
what are the three types of nuclear waste
Low level
Intermediate level
High level
what is low-level nuclear waste and how can it be disposed
- This is waste such as clothing, gloves and tools which may be lightly contaminated
- This type of waste will be radioactive for a few years, so must be encased in concrete and stored a few metres underground until it can be disposed of with regular waste
what is intermediate-level waste and how can it be disposed
- This is everything between daily used items and the fuel rods themselves
- Usually, this is the waste produced when a nuclear power station is decommissioned and taken apart
- This waste will have a longer half-life than the low-level waste, so must be encased in cement in steel drums and stored securely underground
what is high-level nuclear waste
- This waste comprises of the unusable fission products from the fission of uranium-235 or from spent fuel rods
- This is by far the most dangerous type of waste as it will remain radioactive for thousands of years
- As well as being highly radioactive, the spent fuel rods are extremely hot and must be handled and stored much more carefully than the other types of waste
how is high-level waste disposed
- The waste is initially placed in cooling ponds of water close to the reactor for a number of years
- Isotopes of plutonium and uranium are harvested to be used again
- Waste is mixed with molten glass and made solid (this is known as vitrification)
- Then it is encased in containers made from steel, lead, or concrete
- This type of waste must be stored very deep underground
benefits of nuclear power
- Nuclear power stations produce no polluting gases
- They are highly reliable for the production of electricity
- They require far less fuel as uranium provides far more energy per kg compared to coal and other fossil fuels
Purpose of fuel rods
- Enriched with Fuel
- e.g Uranium has a high percentage of fissionable isotopes
Describe the moderation of fission reactors
- During fission, neutrons are released with high energies and must be slowed down by water moderation to maintain the chain reaction
- The first few collisions of a neutron with the moderator transfer sufficient energy to excite nuclei in the moderator with the neutrons being absorbed
- The subsequent collisions of a neutron with the moderator are elastic (momentum and energy are conserved)
- In these subsequent collisions, momentum is transferred to the moderator atoms
- With each collision, the neutron slows down until the average kinetic energy of the neutrons corresponds to that of the moderator nuclei
- Eventually (after about 50 collisions), the neutrons reach speeds associated with thermal random motion (hence the name thermal neutron)
- At these speeds, neutrons can cause fission rather than rebound off of the uranium nuclei
- A moderator with a similar mass to the neutrons is used (e.g water)
What are the conservation of energy and momentum equations during the moderation of a fission reaction
Negatives of nuclear power
- Nuclear accidents are disastrous. Leakage of nuclear isotopes into environment is the main danger.
- Difficult to dispose nuclear waste
- Target for terrorism. Nuclear substances can be used to make nuclear bombs. This is a controversial topic
Describe the setup of the Rutherford scattering experiment
- Evidence for the structure of the atom was discovered by Rutherford from the study of α-particle scattering
- The experimental setup consists of alpha particles fired at thin gold foil and a detector on the other side to detect how many particles deflected at different angles
What were the conclusions made from Rutherford’s scattering and why
- The majority of α-particles went straight through (A)
This suggested the atom is mainly empty space - Some α-particles deflected through small angles
This suggested there is a positive nucleus at the centre (since two positive charges would repel) - Only a small number of α-particles deflected straight back at angles of > 90o
This suggested the nucleus is extremely small and this is where the mass and charge of the atom is concentrated
It was therefore concluded that atoms consist of small dense positively charged nuclei
Describe and explain the ionising/penetrating power of alpha radiation
- Alpha is the most ionising type of radiation
This is due to it having the highest charge of +2e
This means it produces the greatest number of ion pairs per mm in air
This also means it is able to do more damage to cells than the other types of radiation - Alpha is the least penetrating type of radiation
This means it travels the shortest distance in air before being absorbed
Alpha particles have a range of around 3-7 cm in air
Alpha can be stopped by a single piece of paper
Describe and explain the ionising/penetrating power of beta radiation
- Beta is a moderately ionising type of radiation
This is due to it having a charge of +1e
This means it is able to do some slight damage to cells (less than alpha but more than gamma) - Beta is a moderately penetrating type of radiation
Beta particles have a range of around 20 cm - 3 m in air, depending on their energy
Beta can be stopped by a few millimetres of aluminium foil
Describe and explain the ionising/penetrating power of gamma radiation
- Gamma is the least ionising type of radiation
This is because it is an electromagnetic wave with no charge
This means it produces the least number of ion pairs per mm in air
It can still cause damage to cells, but not as much as alpha or beta radiation. This is why it is used for cancer radiotherapy - Gamma is the most penetrating type of radiation
This means it travels the furthest distance in air before being absorbed
Gamma radiation has an infinite range and follows an inverse square law
Gamma can be stopped by several metres of concrete or several centimetres of lead
Describe 2 applications of alpha, beta or gamma radiation in real life
- Smoke detectors:
- Smoke detectors contain a small amount of a weak alpha source
- Within the detector, alpha particles are emitted and cause the ionisation of nitrogen and oxygen molecules in the air
- These ionised molecules enable the air to conduct electricity and hence a small current can flow
- If smoke enters the alarm, it absorbs the alpha particles, hence reducing the current which causes the alarm to sound - Thickness controls:
- Beta radiation can be used to determine the thickness of aluminium foil, paper, plastic, and steel
- The thickness can be controlled by measuring how much beta radiation passes through the material to a Geiger counter
- Beta radiation must be used, because:
Alpha particles would be absorbed by all the materials
Gamma radiation would pass through undetected through the materials
- The Geiger counter controls the pressure of the rollers to maintain the correct thickness
- A source with a long half-life must be chosen so that it does not need to be replaced often
What do the components of the inverse-square law for gamma radiation equation mean
How can the inverse-square law for gamma radiation equation be re-written for radiation at two different points
What are the two sources of gamma radiation
- natural sources
- man made sources
What are the natural sources of gamma radiation
- Radon gas from rocks and soil
Heavy radioactive elements occur naturally in rocks in the ground
One of these (Uranium) decays into radon gas, which is an alpha emitter
This is particularly dangerous if inhaled into the lungs in large quantities - Cosmic rays from space
The sun emits an enormous number of protons every second
Some of these enter the Earth’s atmosphere at high speeds
When they collide with molecules in the air, this leads to the production of gamma radiation
Other sources of cosmic rays are supernovae and other high energy cosmic events - Carbon-14 in biological material
All organic matter contains a tiny amount of carbon-14 - Radioactive material in food and drink
Naturally occurring radioactive elements can get into food and water since they are in contact with rocks and soil containing these elements
What are the man made sources of gamma radiation
- Medical sources
In medicine, radiation is utilised all the time - Nuclear waste
While nuclear waste itself does not contribute much to background radiation, it can be dangerous for the people handling it - Nuclear fallout from nuclear weapons
Fallout is the residue radioactive material that is thrown into the air after a nuclear explosion - Nuclear accidents
Accidents contributed a large dose of radiation into the environment
While these accidents are now extremely rare, they can be catastrophic and render areas devastated for centuries
What are the precautions that must be taken when handling radioactive sources
- Keeping radioactive sources shielded when not in use, for example in a lead-lined box
- Wearing protective clothing to prevent the body from becoming contaminated
- Keeping personal items outside of the room to prevent these from becoming contaminated
- Limiting exposure time so less time is spent with radioactive materials
- Handling radioactive materials with long tongs to increase the distance from them
- Monitoring the exposure of workers, such as radiographers, using detector badges
How is nuclear radiation used in medicine
- Radiation Therapy:
Gamma radiation can be used to destroy cancerous tumours
The gamma rays are concentrated on the tumour to protect the surrounding tissue
Less penetrating beta radiation can be used to treat skin cancer by direct application to the affected area - Radioactive Tracers
Radioisotopes can be used as ‘tracers’ to monitor the processes occurring in different parts of the body - Sterilising Medical Equipment
Gamma radiation is widely used to sterilise medical equipment
What kind of radioactive tracers are used in medicine
Radioactive tracers with a short half-life are preferred because:
- Initially, the activity is very high, so only a small sample needed
- The shorter the half-life, the faster the isotope decays
- Isotopes with a shorter half-life pose a much lower risk to the patient
- The medical test doesn‘t last long so a half-life of a few hours is enough
- e.g Iodine-131 or Technetium-99m
Why is gamma radiation best for sterilising medical equipment
- It is the most penetrating out of all the types of radiation
- It is penetrating enough to irradiate all sides of the instruments
- Instruments can be sterilised without removing the packaging
Why does medical equipment not become radioactive if it has been sterilised using medical equipment
- In order for a substance to become radioactive, the nuclei have to be affected
- Ionising radiation only affects the outer electrons and not the nucleus
- The radioactive material is kept securely sealed away from the packaged equipment so there is no chance of contamination
Radioactive decay def
The spontaneous disintegration of a nucleus to form a more stable nucleus, resulting in the emission of an alpha, beta or gamma particle
What does it mean that radioactive decay is a random process
- There is an equal probability of any nucleus decaying
- It cannot be known which particular nucleus will decay next
- It cannot be known at what time a particular nucleus will decay
- The rate of decay is unaffected by the surrounding conditions
- It is only possible to estimate the proportion of nuclei decaying in a given time period
How can the random nature of radioactive decay be demonstrated
By observing the count rate of a Geiger-Muller (GM) tube:
- When a GM tube is placed near a radioactive source, the counts are found to be irregular and cannot be predicted
- Each count represents a decay of an unstable nucleus
Decay constant def
The probability that an individual nucleus will decay per unit of time
What does it mean when a radioactive sample has a high activity
the number of decays per unit time is very high
What do the components of the radioactive activity equation mean
Unit of radioactive activity and what is it equal to
Becquerels (Bq)
An activity of 1 Bq is equal to one decay per second, or 1 s-1
What does the radioactivity equation show
- The greater the decay constant, the greater the activity of the sample
- The activity depends on the number of undecayed nuclei remaining in the sample
- The minus sign indicates that the number of nuclei remaining decreases with time
Draw and describe the exponential decay curve for radioactivity
What do the components of the number of undecayed nuclei equation mean
What is, and what are the components of, the activity equation (similar to undecided nuclei equation)
What is, and what are the components of, the count rate equation (similar to undecided nuclei equation)
What is the exponential constant
The symbol e represents the exponential constant
It is approximately equal to e = 2.718
Half-life def
The time taken for the initial number of nuclei to halve for a particular isotope
Draw a half-life graph
What does the half-life equation show
How can half life be found from decay curves?
by:
- Drawing a line to the curve at the point where the activity has dropped to half of its original value
- Drawing a line from the curve to the time axis, this is the half-life
Draw the log graph for a decaying radioactive source
What kind of elements are lighter/heavier
lighter elements (with fewer protons) tend to be much more stable than heavier ones (with many protons)
Draw the nuclear stability curve of N against Z
When will a nucleus be unstable
When it has:
- Too many neutrons
- Too many protons
- Too many nucleons ie. too heavy
- Too much energy
Describe the effect of a proton-neutron imbalance on stability
- At a short range (around 1–4 fm), nucleons are bound by the strong nuclear force
- Below 1 fm, the strong nuclear force is repulsive in order to prevent the nucleus from collapsing
- At longer ranges, the electromagnetic force acts between protons, so more protons cause more instability
- Therefore, as more protons are added to the nucleus, more neutrons are needed to add distance between protons to reduce the electrostatic repulsion
- Also, the extra neutrons increase the amount of binding force which helps to bind the nucleons together
Describe how an N-Z graph can be used to determine alpha and beta emitters, and electron capture
Alpha-emitters:
- Occur beneath the line of stability when Z > 60 where there are too many nucleons in the nucleus
- These nuclei have more protons than neutrons, but they are too large to be stable
- This is because the strong nuclear force between the nucleons is unable to overcome the electrostatic force of repulsion between the protons
Beta-minus (β-) emitters:
- Occur to the left of the stability line where the isotopes are neutron-rich compared to stable isotopes
- A neutron is converted to a proton and emits a β– particle (and an anti-electron neutrino)
Beta-plus (β+) emitters:
- Occur to the right of the stability line where the isotopes are proton-rich compared to stable isotopes
- A proton is converted to a neutron and emits a β+ particle (and an electron neutrino)
Electron capture:
- When a nucleus captures one of its own orbiting electrons
- As with β+ decay, a proton in the nucleus is converted into a neutron, releasing a gamma-ray (and an electron neutrino)
- Hence, this also occurs to the right of the stability line where the isotopes are proton-rich compared to stable isotopes
How does an atom with too many neutrons decay
How does an atom with too many protons decay
How does an atom with too many nucleons decay
What are the reasons why an atom might decay
- too many neutrons
- too many protons
- too many nucleons
- too much energy
How does an atom with too much energy decay
Show how alpha and beta radiation, and electron capture can be shown on a graph
Draw the nuclear energy level diagram for molybdenum
What is the equation for finding the distance between the electron fired and the nucleus, in Rutherford’s scattering experiment
What are the advantages and disadvantages of the closest approach method in Rutherford’s scattering
Advantages
- Alpha scattering gives a good estimate of the upper limit for a nuclear radius
- The mathematics behind this approach are very simple
- The alpha particles are scattered only by the protons and not all the nucleons that make up the nucleus
Disadvantages
Disadvantages
- This method does not give an accurate value for nuclear radius as it will always be an overestimate
This is because it measures the nearest distance the alpha particle can get to the gold nucleus, not the radius of it - Alpha particles are hadrons, therefore, when they get close to the nucleus they are affected by the strong nuclear force and the mathematics do not account for this
- The gold nucleus will recoil as the alpha particle approaches
- Alpha particles have a finite size whereas electrons can be treated as a point mass
- It is difficult to obtain alpha particles which rebound at exactly 180°
In order to do this, a small collision region is required - The alpha particles in the beam must all have the exact same initial kinetic energy
- The sample must be extremely thin to prevent multiple scattering
Describe how nuclear radius can be calculated through electron diffraction
What are the advantages and disadvantages of electron diffraction for calculating nuclear radius
What is the equation for Rutherford’s scattering
Draw and describe the graph linking nuclear radius and nucleon number
What do the components of the nuclear radius equation mean
What is, and what are the components of, the nuclear density equation
What does the fact that Nuclear density is significantly larger than atomic density suggest
- The majority of the atom’s mass is contained in the nucleus
- The nucleus is very small compared to the atom
- Atoms must be predominantly empty space
Draw a diagram of electron capture and an electron-proton collision
Is uranium 238 fissionable by thermal neutron
- Uranium 238 is not fissionable by thermal neutrons, but it can undergo fission from fast or high energy neutrons.
- U-238 will instead absorb/scatter neutrons
What is likely to occur to fission products after they have been produce and why
Due to being relatively neutron-rich for their atomic number, many of them quickly undergo beta decay.
